This is, as you say, the problem with mystical experiences that aren’t subjected to the spiritual equivalent of “peer revue”.
This kind of thing is not at all unheard of. Even outside of any disciplined meditative tradition, people stumble upon insights into the Emptiness that transcends self-and-other. This isn’t something that one “learns” from being a Buddhist or Hindu or Taoist; being human is qualification enough.
The rub is that these insights, as transformative and affirming as they may be, open a person up to dangers that they typically can’t see in the afterglow of the insight. The impression of someone in this state is that the ego has been extinguished, and that this all-pervading Emptiness is one’s new reality. In fact, though, the ego will soon reemerge and co-opt even this experience (this is when one begins to think of oneself as “enlightened”), but one will be unable to see this happening. And the Emptiness will become its own form of blindness.
Established traditions know and warn against these dangers, and have mechanisms in place to counteract them, including checking and interventions by experienced practitioners who know this landscape. They will do what’s necessary to keep the person grounded and keep them from clinging to the experience.
A classic example of this was the Japanese Zen master Hakuin. He once wrote in a letter to a friend about one of his first mystical experiences and its aftermath:
“I chanced to hear the sound of the temple bell and I was suddenly transformed. It was as if a sheet of ice had been smashed or a jade tower had fallen with a crash. Suddenly I returned to my senses. I felt then that I had achieved the status of Yen-t’ou [a famous master]... All my former doubts vanished as though ice had melted away. In a loud voice I called: “Wonderful, wonderful. There is no cycle of birth and death through which one must pass. There is no enlightenment one must seek. The seventeen hundred koans handed down from the past have not the slightest value whatsoever.” My pride soared up like a majestic mountain, my arrogance surged forward like the tide. Smugly I thought to myself: “In the past two or three hundred years no one could have accomplished such a marvelous breakthrough as this.”
Shouldering my glorious enlightenment, I set out at once for Shinano. Calling on Master Shoju, I told of my experience and presented him with a verse. The Master, holding my verse up in his left hand, said to me: “This verse is what you have learned from study. Now show me what your intuition has to say,” and he held out his right hand.
I replied: “If there were something intuitive that I could show you, I’d vomit it out,” and I made a gagging sound.
The Master said: “How do you understand Chao-chou’s Mu? [His teacher is testing his insight here with a famous koan]”
I replied: “What sort of place does Mu have that one can attach arms and legs to it?” [His answer shows that he is now stuck in Emptiness]
The Master twisted my nose with his fingers and said: “Here’s someplace to attach arms and legs.” I was nonplussed and the Master gave a hearty laugh. “You poor hole-dwelling devil!” he cried. I paid him no attention and he continued: “Do you think somehow that you have sufficient understanding?”
I answered: “What do you think is missing?”
Then the Master began to discuss the koan that tells of Nan-ch’uan’s death [Implying that Hakuin still needs needs to keep going in his exploration]. I clapped my hands over my ears and started out of the room. The Master called after me, “Hey, monk!” and when I turned to him he added: “You poor hole-dwelling devil!” From then on, almost every time he saw me, the Master called me a “poor hole-dwelling devil.”
Had Hakuin not had the benefit of someone to flush him out of his “hole” and get him back on track, I imagine that he would have ended up very much like your friend.